1. Field of the Invention
The present invention relates to a microswitching device which is manufactured utilizing MEMS technology, and also to a microswitching device manufacturing method utilizing MEMS technology.
2. Description of the Related Art
In the field of portable telephones and other wireless communication equipment, increases in the number of mounted components in order to realize more sophisticated functions have been accompanied by demands for miniaturization of high-frequency circuits and RF circuits. In order to respond to such demands, efforts have been in progress for the miniaturization of various components using MEMS (micro-electromechanical systems) technology.
A MEMS switch is a switching device each of the components of which are formed to be very fine, and has at least one pair of contacts which are mechanically opened and closed to execute switching, and a driving mechanism to achieve mechanical open/close operation of the contact pair. MEMS switches tend to exhibit higher insulating properties in the open state, and a lower insertion loss in the closed state, than such switches as PIN diodes and MESFETs, particularly in high-frequency switching in the GHz range. This is because an open state is achieved through mechanical separation of the contact pair, and because there is little stray capacitance due to the fact that the switching is mechanical. MEMS switches are for example described in Japanese Patent Laid-open No. 2004-1186, Japanese Patent Laid-open No. 2004-311394, Japanese Patent Laid-open No. 2005-293918, and National Publication of Translation for PCT Application No. 2005-528751.
FIG. 14 through FIG. 18 show a microswitching device X2, which is an example of conventional microswitching devices. FIG. 14 is a plane view of the microswitching device X2, and FIG. 15 is a partial plane view of the microswitching device X2. FIG. 16 through FIG. 18 are cross-sectional views along lines XVI-XVI, XVII-XVII, and XVIII-XVIII in FIG. 14 respectively.
The microswitching device X2 comprises a base S2, fixed portion 41, movable portion 42, contact electrode 43, pair of contact electrodes 44 (omitted in FIG. 15), driving electrode 45, and driving electrode 46 (omitted in FIG. 15), and is configured as an electrostatic driving device.
The fixed portion 41 is joined to the base S2 with the boundary layer 47 intervening, as shown in FIG. 16 through FIG. 18. The fixed portion 41 and base S2 are made of single-crystal silicon, and the boundary layer 47 is made of silicon dioxide.
As for example shown in FIG. 14, FIG. 15, or FIG. 18, the movable portion 42 has a fixed end 42a fixed to the fixed portion 41 and a free end 42b, is extended along the base S2, and is surrounded by the fixed portion 41 with a slit 48 intervening. The movable portion 42 is made of single-crystal silicon.
The contact electrode 43 is provided close to the free end 42b on the movable portion 42, as shown in FIG. 15. As shown in FIG. 16 and FIG. 18, each of the pair of contact electrodes 44 is provided standing upright on the fixed portion 41, and moreover has a region opposing the contact electrode 43. Each of the contact electrodes 44 is connected to a prescribed circuit for switching via prescribed wiring (not shown).
The driving electrode 45 is provided over the movable portion 42 and fixed portion 41, as shown in FIG. 15. The driving electrode 46 is provided standing upright such that the two ends are joined to the fixed portion 41 and span the driving electrode 45, as shown in FIG. 17. The driving electrode 46 is connected to ground via prescribed wiring (not shown). These driving electrodes 45 and 46 form an electrostatic driving mechanism.
When a prescribed potential is applied to the driving electrode 45 of a microswitching device X2 configured in this way, an electrostatic attractive force occurs between the driving electrodes 45 and 46. As a result, the movable portion 42 is elastically deformed to the position at which the contact electrode 43 makes contact with both the contact electrodes 44. In this way, the closed state of the microswitching device X2 is achieved. In the closed state, the pair of contact electrodes 44 is electrically bridged by the contact electrode 43, so that current is permitted to pass between the contact electrode pair 44. In this way, for example, a high-frequency signal turn-on state can be achieved.
On the other hand, when the microswitching device X2 is in the closed state, by halting the application of a potential to the driving electrode 45 the electrostatic attractive force acting between the driving electrodes 45 and 46 is annihilated, the movable portion 42 returns to its natural state, and the contact electrode 43 is isolated from the contact electrodes 44. In this way, as shown in FIG. 16 and FIG. 18, the open state of the microswitching device X2 is achieved. In the open state, the pair of contact electrodes 44 are electrically separated, and the passage of current between the contact electrode pair 44 is impeded. In this way, for example, a high-frequency signal turn-off state can be achieved.
FIG. 19 through FIG. 21 show a method of manufacture of a microswitching device X2, as changes in cross-sections equivalent to those of FIG. 16 and FIG. 17. In the manufacture of a microswitching device X2, first a material substrate S2′ such as shown in FIG. 19A is prepared. The material substrate S2′ is a so-called SOI (silicon on insulator) substrate, and has a stacked structure comprising a first layer 51, second layer 52, and intermediate layer 53 between these. The first layer 51 and second layer 52 are made of single-crystal silicon, and the intermediate layer 53 is made of silicon dioxide.
Next, as shown in FIG. 19(b), sputtering is used to form a conductive film 54 on the first layer 51. The conductive film 54 has a uniform thickness of 0.75 μm.
Next, as shown in FIG. 19(c), the resist patterns 55 and 56 are formed on the conductive film 54. The resist pattern 55 has a pattern shape corresponding to the contact electrode 43. The resist pattern 56 has a pattern shape corresponding to the driving electrode 45.
Next, as shown in FIG. 20(a), the resist patterns 55 and 56 are used as masks to perform etching on the conductive film 54, in order to form the contact electrode 43 and driving electrode 45 on the first layer 51. The contact electrode 43 and driving electrode 45 formed in this way have the same thickness of 0.75 μm.
Next, after removing the resist patterns 55 and 56, etching on the first layer 51 is performed to form the slit 48, as shown in FIG. 20(b). Specifically, photolithography is used to form a prescribed resist pattern (not shown) on the first layer 51, after which the resist pattern is used as a mask to perform etching on the first layer 51. In this process, the fixed portion 41 and movable portion 42 are patterned and formed.
Next, as shown in FIG. 20(c), a sacrificial layer 57 is formed on the substrate S2′, on the side of the first layer 51, so as to fill the slit 48. The sacrificial layer 57 is made of silicon dioxide. In this process, the sacrificial layer material is deposited on a portion of the side walls of the slit 48 as well, to fill the slit 48. By adjusting the thickness of the sacrificial layer 57 formed in this process, it is possible to adjust the isolation distance in the open state between the contact electrodes 43 and 44 and between the contact electrodes 45 and 46 in the microswitching device X2 obtained. The thickness of the sacrificial layer 57 is set to 5 μm or less. This is because if the thickness of the sacrificial layer 57 exceeds 5 μm, then internal stresses occurring within the sacrificial layer 57 may result in improper warping of the material substrate S2′, and cracks tend to occur in the sacrificial layer 57.
Next, as shown in FIG. 21(a), the sacrificial layer 57 is patterned to form opening portions 57a and 57b. The opening portion 57a is provided to expose the area of the fixed portion 41 to which the contact electrode 44 is to be joined. The opening portion 57b is provided to expose the area of the fixed portion 41 to which the driving electrode 46 is to be joined.
Next, a prescribed resist pattern (not shown) formed on the sacrificial layer 57 is used as a mask to perform electroplating, to form the pair of contact electrodes 44 and the driving electrode 46, as shown in FIG. 21(b).
Next, as shown in FIG. 21(c), wet etching is performed to remove the sacrificial layer 57 and a portion of the intermediate layer 53. In this etching process, first the sacrificial layer 57 is removed, and then a portion of the intermediate layer 53 is removed from the location bordering the slit 48. This etching is halted after an appropriate gap is formed between the entirety of the movable portion 42 and the second layer 52. In this way, the above-described boundary layer 47 is formed to remain in the intermediate layer 53. The second layer 52 forms the base S2. By means of the above processes, an electrostatic-driving type microswitching device X2 is formed.
A small driving voltage is one characteristic which is strongly demanded of an electrostatic-driving type switching device. In order to reduce the driving voltage of the microswitching device X2, it is useful to make the movable portion 42 thin and to design the movable portion 42 to have a small spring constant.
On the other hand, a low insertion loss for signals passed by the contact electrodes in the closed state is generally demanded of switching devices. In order to lower the insertion loss of the switching device, it is useful to set make the contact electrodes thick and design the contact electrodes to have low resistance.
However, in a microswitching device X2 of the prior art, there is a tendency toward increasing difficulty in reducing the resistance of the contact electrode 43. This is because in the microswitching device X2, the contact electrode 43 cannot readily be made thick due to the need to lower the driving voltage, as described above.
As explained above referring to FIG. 19(b), (c), the contact electrode 43 and driving electrode 45 are formed by patterning from the conductive film 54 of uniform thickness formed on the first layer 51, and have the same thickness. As a result, if a large thickness is chosen for the contact electrode 43 so as to reduce the resistance of the contact electrode 43, the driving electrode 45 also has a large thickness. The larger the thickness of the driving electrode 45, the larger is the internal stress which occurs so as to shrink the driving electrode 45, and consequently the action of the internal stress causes the movable portion 42 to be deformed improperly, tending to result in the problem of warping on the side of the contact electrode 44 and driving electrode 46. Such warping of the movable portion 42 impedes the switching function of the microswitching device X2, and induces degradation of various characteristics, and so is undesirable. For example, due to warping of the movable portion 42, there are cases in which the contact electrodes 43 and 44 come into contact even when there is no driving (when no voltage is applied across the driving electrodes 45 and 46), and there are cases in which the driving electrodes 45 and 46 are always in contact. In order to avoid such states, it is necessary to reduce the thickness of the driving electrode 45 and of the contact electrode 43 formed to the same thickness as the driving electrode 45, relative to the thickness of the movable portion 42, which is set to a prescribed small value from the standpoint of reducing the driving voltage. Specifically, it is necessary to make the driving electrode 45 and contact electrode 43 thin, so as to suppress warping of the movable portion 42, within the limits of the isolation distance between the movable portion 42 and the contact electrode 44 and the isolation distance between the movable portion 42 and the driving electrode 46, which can be realized utilizing the sacrificial layer 57 formed to a thickness of 5 μm or less as described above.
Thus when using the technology of the prior art for microswitching devices, there are cases in which it is difficult to realize a sufficiently low-resistance contact electrode and reduce insertion losses while keeping the device driving voltage low.